1. Field of the Invention
The invention in general relates to the transmission of very low frequencies and particularly to an arrangement which utilizes an aerostat tether for the antenna of the communication system.
2. Background Information
For military or civil emergency situations, a need exists for a relatively small, portable VLF communication system which may be transported to a site and quickly placed into operation. In view of the relatively long antenna structure required for a VLF communication system, it becomes impractical to transport a tower structure which may be thousands of feet long. Accordingly, proposals have been made for using the tether of an aerostat system as the VLF antenna. In this manner, the aerostat may be deployed to an operational altitude measured in thousands of feet, when in a deployed condition, while in a transportable condition, the tether may be wound upon a winch and the aerostat itself may be in a deflated condition.
One such system is described in U.S. Pat. No. 4,476,576 which illustrates an aerostat tether wound upon a winch mechanism with the conductors of the tether being connected to a slip ring assembly. An intelligence signal to be transmitted is coupled to the slip ring assembly via an antenna tuner and a transformer. In view of the fact that the arrangement applies a very high voltage to the winch mechanism, a requirement existed for complete electrical isolation of the winch mechanism from a carrier vehicle so as to obviate a potentially dangerous situation. It is more desirable to completely eliminate any potential hazard to operating personnel.
In another system actually used by the assignee of the present invention, a mooring system was utilized and included a horizontally movable boom having at the end thereof a pulley or sheave member and the tether for the aerostat passed from the winch mechanism on a carrier structure, through the boom and around the sheave to the deployed aerostat. The tether included a dual concentric metallic braiding and an electrical connection from the transmitter was made to the braiding. Such electrical connection however required an objectionable stripping operation of the braiding and in addition, the apparatus utilized an objectionably large and costly inductor for tuning purposes.
The present invention provides a VLF communication system utilizing a tether of an aerostat and eliminates the objectionable features of the previous tethered aerostat systems.